Tachometric Surveying

Tacheometry is a branch of surveying in which horizontal and vertical distances are determined by taking angular observations with an instrument known as a tacheometer.
The chaining operation is completely eliminated in such a survey.
Tacheometric surveying is adopted in rough and difficult terrain where direct

levelling and chaining are either not possible or very tedious.

It is also used in location survey for railways, roads, reservoirs, etc.
Though not very accurate, tacheometric surveying is very rapid, and a reasonable contour map can be prepared for investigation works within a short time on the basis of such survey.

Table of Contents

Instruments used in Tacheometry

The Tacheometer: It is nothing, but a transit theodolite fitted with a stadia diaphragm and an analytic lens.

The Levelling Staff or Stadia Rod: For short distances, ordinary levelling staves are used. The levelling staff is normally 4 m long and can be folded into three parts. The graduations are so marked that a minimum reading of 0.005 or 0.001 m, can be taken.

For long sights, a specially designed graduated rod is used, which is known as a stadia rod.

It is also 4 m long and may be folded. The graduations are comparatively bold and clear and the minimum reading that can be taken as 0.01 m.

Principle of Tacheometry

The principle of tacheometry is based on the property of isosceles triangles, where the ratio of the distance of the base from the apex and the length of the base is always constant.

Methods of Tacheometry

Tacheometry involves mainly two methods:

The stadia method
The tangential method

1. The Stadia Method

In this method, the diaphragm of the tacheometer is provided with two stadia hairs (upper and lower).
Looking through the telescope the stadia hair readings are taken.
The difference in these readings gives the staff intercept.
To determine the distance between the station and the staff, the staff intercept is multiplied by the stadia constant (i.e., multiplying constant, 100).
Both horizontal and vertical distances can be calculated.

Case I When line of sight is horizontal, and staff is held vertical.

Case IIWhen line of sight is inclined, and staff is held vertical.

Case III When line of sight is inclined, but staff normal to it.

2. The Tangential Method

In this method the telescope doesn’t contain all stadia lines, only a single horizontal line is provided in the telescope.
Horizontal and vertical distances are calculated by measuring vertical angles (angle of elevation and angle of depression).
Not commonly used method.